Phospholipid head groups, functions

Phospholipid(s) 379, 380,382 - 387, 392. See also Specific substances bilayer diagram 391 head groups, functions of 396 inverted hexagonal phase 397 31P NMR 397 non-bilayer structures 397 Phosphomannomutase 654 Phosphomutases 526 Phosphonamidate 626s... 

The mechanism of inhibition in near-atmospheric CO2 is related to both metabolism and the function of the plasma membrane (24). The permeability of the cellular membrane to dissolved, unhydrated CO2, a neutral molecule, creates disorder and alters the membrane fluidity even at near-atmospheric pressures. However, unlike typical small anaesthetic molecules which alter membrane fluidity, the water permeability of the cell decreases upon contact with CO2 (24). Jones and Greenfield (24) suggest that this unique property of CO2 inhibition is due to the presence of the bicarbonate ion, which may act on the phospholipid head groups and the proteins near the surface of the membrane to alter the surface charge of the cell. 

There are other ways in which the lateral organization (and asymmetry) of lipids in biological membranes can be altered. Eor example, cholesterol can intercalate between the phospholipid fatty acid chains, its polar hydroxyl group associated with the polar head groups. In this manner, patches of cholesterol and phospholipids can form in an otherwise homogeneous sea of pure phospholipid. This lateral asymmetry can in turn affect the function of membrane proteins and enzymes. The lateral distribution of lipids in a membrane can also be affected by proteins in the membrane. Certain integral membrane proteins prefer associations with specific lipids. Proteins may select unsaturated lipid chains over saturated chains or may prefer a specific head group over others. 

Covalent attachment of proteins to the surface of liposomal bilayers is done through reactive sites created on the head groups of phospholipids with the intermediary use of a crosslinker or other activating agent. The lipid functional groups described in Section 1 of this chapter are modified according to the methods discussed in Section 2 to be reactive toward specific target... 

Liposomes are characteristic hollow spherical aggregates or vesicles that form spontaneously when phospholipids are dispersed in water. This is a function of their low solubilities in both oil and water and results from the hydrophobic nature of the twin acyl tails and the strongly hydrophilic polar head group which are the main characteristics of phospholipids (Figure 9.1). As a result, phospholipid molecules, when dispersed in water form double layers where the phospholipids align themselves, tail-to-tail and head-to-head (Figure 9.5)... 

The amount of disorder in the fatty acid side chains in a phospholipid bilayer, as a function of temperature. The side chains become much more disordered as the temperature increases through the 7m. The solid curve represents a bilayer that does not contain cholesterol the dotted curve, a bilayer with the same phospholipid plus about 25% cholesterol. The amount of random, disorderly motion of the fatty acid chains can be measured quantitatively by deuterium- or l3C-NMR. At any given temperature, the disorder is greater near the tips of the chains (toward the middle of the bilayer) than it is close to the head-groups. 

In some cases the functions of phospholipases in cells are purely degradative and result in the release of the phospholipid components (fatty acids, glycerol, phosphate, and head-groups). But in many cases phospholipases have important roles in synthesis and regulation. For example, we have seen how phospholipase A2 catalyzes the first step in the remodeling of phosphatidylcholine to the surfactant... 

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